1. Field of the Invention
This invention relates to a so-called TTL type focus detecting apparatus which accomplishes in-focus detection by the use of an imaging light flux from a photo-taking lens, for example, in a single lens reflex camera.
2. Description of the Prior Art
The assignee of the present invention has proposed, in Japanese Patent Application No. 150520/1982 filed on Aug. 30, 1982, etc., various novel constructions of a focus detecting apparatus in which an imaging optical system is disposed rearwardly of the predetermined imaging plane of an objective lens to thereby form a plurality of object images on the basis of lights passed through different portions of the pupil of the objective lens and each of the object images is detected by a line sensor comprising a plurality of photoelectric conversion elements to thereby detect the focus condition of the objective lens from the amount of deviation of each object image.
Such a focus detecting apparatus effects focus detection only on the basis of the reflected natural ray from an object which is the focus target and is therefore called the passive type. In the focus detecting apparatus of such passive type, when the object brightness is sufficiently high, the apparatus performs well, but when the object brightness becomes lower than a certain value, the quantity of light entering the sensor is insufficient and thus, the operation of the apparatus becomes unstable. For this reason, in cameras having such apparatus, there is known a method of providing an auxiliary light source on the camera body, which auxiliary light source emits light and illuminates the object to be photographed when the object brightness is lower than a limit value, thereby yielding sufficient object brightness to such apparatus. However, where visible ray is utilized as the auxiliary light, there occurs no problem, whereas where infrared ray is utilized as the auxiliary light, there is a great problem. That is, it is difficult to realize a practical focusing optical system which will make compatible the focusing optical system using natural ray for a bright object to be photographed and the focusing optical system using infrared ray for a dark object to be photographed.
Generally, in the photo-taking lens of a camera, there is a difference between the focus position for visible ray and the focus position for infrared ray and well-focused photographs cannot be taken unless in-focus detection is effected with visible ray and infrared ray distinguished from each other. As the focus detecting apparatus which effects in-focus detection with visible ray and infrared ray distinguished from each other in this manner, there are known two types, i.e., the type in which two kinds of optical system for visible ray and infrared ray are prepared and the type in which an infrared cut filter is put into and out of a focusing optical system to thereby realize in-focus detection for two kinds of light. In the former method, two kinds of optical system must be prepared and in the latter method, a mechanism for putting the infrared cut filter into and out of the focusing optical system is necessary. None of these two methods may be said to be practical.
Also, where the aforedescribed focus detecting apparatus is applied to an optical instrument such as a single lens reflex camera in which photo-taking lenses having different brightnesses (F No.) are selectively mounted, it is known that if the passage range of the imaging light flux directed to the sensor is set to the outer peripheral portion of the pupil of a light (small in F No.) photo-taking lens in order to increase the amount of deviation of each image corresponding to the focus condition of the photo-taking lens to thereby enhance the focus detection accuracy, when a dark (great in F No.) photo-taking lens is mounted in such camera, part of the imaging light flux directed to the sensor is eclipsed andd the focus detection accuracy during the use of the dark lens is remarkably reduced. For this reason, in such a focus detecting apparatus according to the prior art, two imaging lenses for forming an image on the basis of the light flux from the center of the pupil of the photo-taking lens and two imaging lenses for forming an image on the basis of the light flux from the outer peripheral portion of the pupil of the photo-taking lens are provided rearwardly of the predetermined imaging plane of the photo-taking lens, whereby the image used for focus detection is changed over in conformity with the brightness of the photo-taking lens. Also, in the aforementioned patent application of the assignee of the present invention, it is proposed to provide an imaging lens, a wedge-prism for dividing the imaging light flux from the center of the pupil of the photo-taking lens and a wedge-prism for dividing the imaging light flux from the outer peripheral portion of the pupil of the photo-taking lens, instead of providing four imaging lenses, thereby enabling the formation of an image corresponding to the brightness of each photo-taking lens.
However, where such a focus detecting apparatus is incorporated in a small optical instrument such as a camera, it becomes necessary to shorten the distance from the predetermined imaging plane of the photo-taking lens to the sensor as much as possible. Therefore, for example, where a wedge-prism is employed, the vertical angle of the wedge thereof must be made great in order that the images based on the light fluxes divided by the wedge-prism may not overlap each other. For this reason, in such an apparatus, the optical aberration of the focusing optical system becomes great and this leads to an unsatisfactory image formed on the sensor. This in turn leads to reduced focus detection accuracy. A similar problem will also occur in the former method in which a plurality of imaging lenses are provided.